The present invention relates to a system for access to an ISDN (Integrated Services Digital Network) from a packet data handling system such as a packet data terminal, a LAN (Local Area Network) or the like.
In the ISDN system, a digital subscriber line (DSL) extends from an exchanging office to a user's area. The DSL terminates to a network termination (NT) unit. A plurality of terminal equipments (TEs) are connected to the NT unit and are, therefore, connected to the DSL through the NT unit. As TEs, there are known a digital telephone set or sets (shich will only be called telephone sets hereinafter), a facsimile set or sets, a personal computer set or computer sets, and the like.
The CCITT (International Telegraph and Telephone Consultative Communication) recommendations I series prescribes protocol of user-network connection or interface in connection with seven classified layers which are numbered by 1 through 7. For example, layer 1 is directed to physical connecting conditions such as transmission rate, number of pins on connectors and others, layer 2 being for data link, layer 3 for call control from a call setup to call release completion.
According to the I series, communication information and control signals are transmitted through different channels which are multiplexed in a time division fashion as a time division multiplex (TDM) signal. The TDM signal comprises two communication information channels of a transmission rate of 64 kbps which are separately called B1 and B2 channels and a single control channel of a transmission rate of 16 kbps which is called D channel.
When the user has a LAN, it is often required to access to the ISDN from the LAN. In a conventional system for access from the LAN to the ISDN, a virtual circuit is set up on a particular one of B1 and B2 channels through the packet switch between the ISDN access system and a called party by the data link establishment and the call control on the D channel in response to a call request from the LAN. Then, packet data from the LAN are converted in protocol at a gateway circuit and are, thereafter, transmitted through the virtual circuit to the called party. The virtual circuit is maintained until disconnection or clear request is generated from the LAN.
Use of the packet switch in the virtual circuit is economical when a small number of data packets are transmitted for a unit time, but degrades throughput of the data when an increased number of data packets are transmitted for a unit time.
By indicating the line switch in the call control in response to the call request, the virtual circuit can be set up through the line switch. Use of the line switch improves the throughput of the data but is not economical when a decreased number of data packets are transmitted for a unit time.
In response to increase and decrease of number of data packets to be transmitted for a unit time, it is possible to clear a current virtual circuit using one of the line switch and the packet switch so as to set up another virtual circuit using another switch. However, it takes time for clearing and setting up the current and the other virtual circuits, respectively, so that throughput of the data is also degraded.